Photographic cameras have been in widespread use for quite some time. Basically, such a camera operates by exposing a portion of a light sensitive media, i.e. a frame of film, for a pre-defined period of time to scene illumination. The light is focused on the frame through a lens that has an aperture of a given, often variable, size. A shutter, situated behind the lens and in front of the film, opens for a selected period of time in order to permit the light to transit therethrough, illuminate and expose the film. As a result of being properly exposed and subsequently developed, the film undergoes a photochemical process, on a two-dimensional basis throughout the frame, that locally varies the optical transmissivity of each portion of the frame in proportion to the amount of illumination that reaches that portion of the frame from a corresponding portion of the scene, thereby producing, depending upon whether reversal or negative film is used, either a two-dimensional positive or negative optical image of the scene. As such, tonal variations that appeared in the scene are captured in the frame of the film. Photographic prints are often made from negatives, while transparencies (commonly referred to as "slides") are made from positives.
Though this overall process, which relies on the use of silver halide has a photosensitive reagent in film, has basically remained unchanged over many years, this process is highly non-linear and subject to a great many variables which significantly complicate its use. In particular, exposure (E) is defined, under a standardized definition, as being a product of the illuminance (I) multiplied by the time (t) during which the film is exposed to this illumination. In this regard, see specifically ANSI (American National Standards Institute) standard PH 3.49-1971 "American National Standard for General Purpose Photographic Exposure Meters" (re-affirmed in its entirety with no modifications in 1987 as ANSI standard PH 3.49-1987) [hereinafter referred to as ANSI standard 3.49-1987], and also ANSI standard PH 2.7-1986 "American National Standard for Photography--Photographic Exposure Guide " and specifically page 13 thereof. In a camera, the combination of two settings, namely lens aperture (size of the lens opening) and shutter speed (time during which the shutter remains open), primarily defines a particular exposure. Unfortunately, lens aperture and shutter speed define more than just an amount of exposure, these settings also dramatically affect picture (hereinafter including both prints and transparencies) quality and must be judiciously chosen in each photographic situation; otherwise, a picture (also referred to hereinafter as an image) having inferior quality will result.
To illustrate the variability among photographic parameters and the difficulties in choosing appropriate lens aperture and shutter speed settings, consider for the moment a particular scenario that often occurs and presents significant challenges to a photographer: photographing a scene in relatively low light conditions with sufficient depth-of-field to cover a desired subject. In dealing with a low light situation, a photographer, particularly an amateur, might, at first, open the lens aperture to its maximum value in order to pass as much light as possible therethrough to the film. A suitable shutter speed would then be chosen based on scene luminance, typically using an indication provided by an internal light meter located in the camera. Unfortunately, such an approach might fail. Specifically, while, the lens aperture size specifies the amount of light that instantaneously strikes the film, this size also defines the so-called depth-of-field in the resulting photographed image, i.e. a range of minimum to maximum camera-to-subject distances in which objects located therein will be perceived in the image as being sharp and in-focus. As the aperture size of a given lens increases, i.e. the lens is opened and its so-called "f" number decreases, which ordinarily occurs in low light conditions, the depth-of-field produced by that lens correspondingly decreases. Accordingly, with certain subject thicknesses, the depth-of-field that results from a maximum lens aperture exposure may be too restricted to fully cover the entire subject. Thus, portions of the subject located at camera-to-subject distances that are outside the range specified by the depth-of-field for the given lens aperture, i.e. too close to or too far from the camera, will appear out-of-focus in the resulting photographed image. Therefore, in order to provide an appropriate depth-of-field to cover the entire subject, a smaller lens aperture than the maximum available size must be used along with a slower shutter speed to generate a sufficient exposure. Unfortunately, a photographer is often unable to steadily hold a hand-held camera for times typically in excess of, for example, 1/50th of a second for a 50 mm lens. Hence, as the shutter remains open for increasingly longer periods of time, the camera becomes increasingly sensitive to camera shake or subject motion which, when it occurs, blurs and ruins the entire picture. Therefore, to reduce the incidence of noticeable image blur, particularly resulting from camera shake, shutter speeds equal to or slower than of 1/50th second for use with a 50 mm lens should be avoided for use in a hand-held camera. Consequently, other techniques, such as mounting the camera on a tripod or using an auxiliary light source, e.g. a so-called flash unit, or higher speed film, i.e. a more sensitive film, are often required in order to provide acceptable combinations of lens aperture openings and shutter speeds in low light conditions. Unfortunately, a flash unit or a tripod may not be currently available. Also, films that are increasingly sensitive tend to produce pictures that exhibit increasing graininess, thereby adversely impacting the quality of the picture. With this scenario and in the absence of having a flash unit, a tripod or the ability to change film for use in photographing a particular low light scene, the photographer may not be able to select shutter speeds and lens aperture sizes that will produce a picture of optimum quality. Instead, the photographer is forced to accept compromise settings which will likely produce a picture of sub-optimum quality, such as being under-exposed, but, owing to the latitude in the performance of the film, will hopefully still exhibit sufficient quality to still be acceptable to a viewer. To a certain extent, the developing process can compensate (through so-called "forcing") for under-exposure conditions, though the ability to do so and still provide pictures of sufficient quality depends upon the subject matter in the scene and hence can be rather limited. In this regard, see D. M. Zwick, "The Technical Basis of Photographic Speed Determination or What is a Normal Exposure", SMPTE Journal, Vol. 88, No. 8, Aug. 1979, pages 533-537 (hereinafter referred to as the Zwick publication) and specifically pages 536-537 thereof.
In certain extreme situations with worsening exposure conditions than that illustratively described above, the lighting conditions may, for all practical purposes, totally frustrate the ability of even a skilled photographer to produce a picture of merely acceptable quality. In these situations, photography would be essentially impossible. For example, consider the same low-light scenario above but where the photographer desires to use a lens that has a relatively large focal length, e.g. a telephoto lens, to capture a scene. For a given film size, the depth-of-field varies in proportion to the square of the focal length of the lens and hence significantly decreases with increases in focal length. Therefore, the depth-of-field provided by such a lens, for certain lens apertures, may not meet the scene requirements. Large focal length lenses also tend to be bulky, massive and relatively heavy and thus, once mounted to a hand-held camera, are hard to hold steady for even moderate shutter speeds, such as 1/30 or 1/60th of a second. Accordingly, to avoid significant camera shake, the slowest shutter speed at which these lenses can be used, without a tripod, is often quite limited. Moreover, since physical limitations on lens size often prevent a large focal length lens from being constructed with large lens aperture sizes, this forces the use of increasingly long shutter speeds to achieve a proper exposure under low-light conditions and exacerbates the need to use other techniques, such as a tripod, auxiliary light source or a faster speed film, to provide usable lens aperture and shutter speed settings that will provide a proper exposure. In the absence of using a tripod or an auxiliary light source, which--owing to the amplitude fall-off as the inverse square of distance to the subject--becomes ineffective at large subject-to-camera distances, or the ability to change to and/or even the availability of sufficiently fast films that exhibit low graininess during the printing process, low-light photography with large focal length lenses is oftentimes practically impossible.
Therefore, as one can now appreciate, even a skilled photographer often experiences difficulties in choosing the proper photographic settings under certain lighting conditions, e.g. lens aperture and shutter settings, selection of lens focal length, use and amount of flash illumination. While certain lighting conditions are so extreme that they simply can not be handled by even a professional photographer, the vast majority of scene lighting conditions fortunately do not fall in this category. Nevertheless, some of these latter conditions often present sufficient difficulties to effectively frustrate the ability of an amateur photographer to take a picture of acceptable quality. In fact, for many inexperienced amateurs, choosing lens aperture size and shutter speed settings amounts to little more than mere guesswork, through which the probability is high that the amateur will select wrong settings and quickly become frustrated. Frustration, if it occurs sufficiently often, leads to dis-satisfaction, which in the context of an amateur photographer often means that the photographer will simply stop taking pictures and turn to other leisure activities which he or she believes to be less trying and more satisfying than photography. Since amateur photographers constitute a major portion of the photographic market, including both equipment and film, their continued satisfaction is essential to the photographic industry.
Having recognized this fact, the art has for many years pursued a goal of developing a camera that, over its lifetime, will produce more pictures that exhibit at least an acceptable and preferably higher level of quality than those resulting from cameras heretofore in use while, at the same time, relieving the photographer of the tedium and difficulty associated with choosing the photographic settings appropriate to a current lighting condition.
Hence, over the years, considerable activity has occurred in the art to provide cameras that automatically select a lens aperture size and/or shutter speed appropriate for a current scene being photographed. While these attempts have resulted in cameras of increasing sophistication and improved performance, each of these attempts suffers one or more drawbacks which limits its attractiveness.
For example, U.S. Pat. No. 3,917,395 (issued to F. T. Ogawa on Nov. 4, 1975) describes one approach at providing an automatic camera. Here, a camera relies on using an electronic circuit and associated electro-mechanical drive mechanisms for invoking and controlling each one of a sequence of photographic operations required by the camera to take a picture. Unfortunately, this aperture appears to require a photographer to manually select an appropriate lens aperture size. With this selection, the circuitry attempts to control shutter speed, determine if a flash is necessary and, if so, and fire the flash unit. Accordingly, while the photographer is advantageously relieved of determining an appropriate shutter speed and whether flash is necessary, improper depth-of-field could readily result in a significant number of pictures, particularly those taken under various low-light conditions, that exhibit unacceptable quality.
Another illustrative approach at providing an automatic camera is described in U.S. Pat. No. 4,103,307 (issued to N. Shinoda et al on Jul. 25, 1978 and hereinafter referred to as the '307 shinoda et al patent). Here, a microcomputer is used within a camera to provide several different modes of automated photography; namely, shutter priority mode (where the photographer manually selects the shutter speed and the camera selects the lens aperture size), aperture priority mode (where the photographer manually selects the lens aperture size and the camera selects the shutter speed), program mode (where the camera selects both the shutter speed and lens aperture size) and manual mode (where the photographer manually selects both the shutter speed and lens aperture size). During camera operation in a non-manual mode, the microcomputer computes the appropriate selection(s) based upon film speed and scene lumination. Unfortunately, since the microcomputer uses scene luminance, but not scene content, in determining exposure settings during program mode and shutter priority modes of operation, the microcomputer can generate a lens aperture size setting that, in many instances, is not likely to provide sufficient depth-of-field for a current scene. Similarly, an amateur photographer who operates the camera in the aperture priority mode may often manually select a lens aperture size that provides insufficient depth-of-field. In the shutter priority mode, the photographer can manually set minimum and maximum lens aperture limits. In this instance, the microcomputer will provide suitable indications in the viewfinder of the camera to alert the photographer when the computed lens aperture size is beyond the set limits, thereby requiring the photographer to manually change the lens aperture size accordingly. However, this need to manually set and/or subsequently manually adjust the exposure settings of the camera places an added burden on the photographer, one which is best avoided in cameras destined for amateur photographers.
An additional illustrative approach is described in U.S. Pat. No. 4,309,089 (issued to D. M. Harvey on Jan. 5, 1982, assigned to the present assignee and hereinafter referred to as the '089 patent). Here, the camera contains a microcomputer which uses scene lumination and exposure latitude information of the film to compute a range of acceptable exposure values (EV values). The current exposure value corresponding to the specific lens aperture size and shutter speed at which the camera is presently set is compared to the range. If the comparison reveals that the specific exposure value lies within or outside of the range, then the microcomputer provides an appropriate indication to the photographer to either "validate" the settings for the prevailing scene lumination or inform the photographer to choose other settings to photograph the scene. Unfortunately, the photographer is required to manually choose both the lens aperture size and shutter speed settings and to manually change them, if necessary. This places a significant and undesirable burden on a photographer, which might be best avoided if the camera is destined for the amateur market.
While these approaches in the art clearly teach that image quality can be improved by increasing camera sophistication through use of an internal microcomputer for scene measurement, and exposure determination and control, all of these approaches share one significant disadvantage: they all appear to require the photographer to manually intervene in some fashion in order to achieve optimum image quality. This intervention takes the form of either manually providing an initial exposure setting, e.g. a lens aperture size that satisfies depth-of-field requirements, or subsequently making manual corrections, as indicated by the microcomputer, to these settings in order to provide a proper exposure. As such, these prior art approaches appear to universally fail to provide truly automatic control over a camera--control that is necessary to substantially, if not completely, eliminate guesswork by amateur photographers and further heighten overall image quality.
Having recognized this failing in the art, U.S. Pat. No. 4,785,323 (issued to C. S. Bell on Nov. 15, 1988, also assigned to the present assignee and hereinafter referred to as the '323 Bell patent) describes automatic exposure control apparatus for use in a camera that through measurement of scene parameters, such as scene lumination and subject-to-camera distance, attempts to satisfy depth-of-field requirements and reduce image blur. In particular, this apparatus first determines various combinations of appropriate lens aperture size and shutter speed settings based upon measured scene lumination. Thereafter, using measured subject-to-camera distance, the apparatus selects one of these combinations that provides the required depth-of-field. As the subject moves farther from the camera, larger lens aperture sizes and increased shutter speeds are selected in an effort to maintain the needed depth-of-field while reducing the likelihood of image blur resulting from either camera shake and/or subject motion. By minimizing image blur, this apparatus can advantageously provide a perceptible increase in overall image sharpness though at the expense, in certain instances, of compromising image exposure.
Nevertheless, further improvements in overall image quality can still be made even if image blur is minimized for a given scene. In this regard, it has been well known for some time in the art, especially by professional photographers, that most films, particularly negative print film and to a much lesser extent reversal films, possess some latitude with respect to exposure. In fact, the curve of log exposure (log E) vs. density for each layer of a color negative film generally remains at a minimum value (i.e. not latent image is formed) until a given minimal amount of exposure is reached at which point the curve begins to increase. The curve then linearly increases in density throughout a range of increasing log exposure values until point is reached at which a shoulder exists in the curve. The curve then exhibits a fairly broad plateau at which image density remains constant or slightly increases throughout several values of increasing log exposure. In addition, with certain film types, the graininess exhibited by the layer tends to decrease with increasing exposure, and increasingly sharp images may occur at increasing exposure levels. The ISO (ASA) film speed is defined from the exposure necessary to produce a specific value of image density on each layer of the film. Typically, the lowest log exposure value on this curve that will provide an "excellent" quality image in terms of faithful tone (color) reproduction defines a so-called normal exposure point. The normal exposure point is not a region but rather by a single point on the log exposure vs. density curve for the film. With these definitions in mind, lens aperture and shutter speed settings, that will produce an exposure at the ISO normal exposure point, can then be readily determined by substituting the values for the ISO (ASA) film speed and scene luminance into an ISO standard metering equation and calculating a result. See, specifically, ANSI standard PH 2.27-1988 "American National Standard for Determination of ISO (ASA) Speed of Color Negative Films for Still Photography" and ISO standard 588-1979, with the former ANSI standard adopting the latter ISO standard for determining the film speed; and the ANSI standard 3.49-1987, particularly page 21 thereof for the ISO standard metering equation; as well as the Zwick publication. For ease of reference, the pertinent standards will be referred to hereinafter as the "ISO/ANSI exposure standards" with an exposure defined by these standards being referred to hereinafter as synonymously either an ISO "standard" or "normal" exposure and the normal exposure point being referred to as the ISO normal exposure point. From these definitions and use of the standard metering equation, the ISO normal exposure point occurs at higher density and exposure values on the log exposure vs. density curve than those for the ISO (ASA) speed point. Inasmuch as linear response can still be had at each of several stops of over-exposure from the ISO normal exposure point and yield improved image quality, professional photographers often, depending upon lighting conditions, intentionally over-expose negative film, beyond that specified by its ISO normal exposure point. Unfortunately, those automated cameras that utilize film speed in determining exposure settings, such as that described above in the '323 Bell, the '307 Shinoda et al and the '089 Harvey patents, do not automatically take into account that, for certain films, image quality can actually be improved through intentional over-exposure. Nevertheless, the art has recognized, as disclosed in illustratively U.S. Pat. No. 4,598,986 (issued to K. Shiratori et al on Jul. 8, 1986 and hereinafter referred to as the '986 Shiratori et al patent) the utility of including a manual adjustment into an automated camera that permits a photographer to manually shift the exposure settings in order to intentionally over- or under-exposure a photographed image. The amount of the shift can either be manually set by the photographer or read by the camera from encoded information stored in metallic patterns (the so-called "DX" code) situated on an outside surface of a film canister. Unfortunately, this arrangement relies on the photographer, not the camera, to make the determination of when the film is to be over-exposed. Since many amateurs are simply not familiar with film exposure characteristics and to avoid what they believe to be a risk, here arising from simple ignorance, of ruining a picture, these amateurs, including those using the camera described in the '986 Shiratori et al patent, are not likely to intentionally over-expose a picture even in those situations where, in fact, image quality could be improved by doing so.
Consequently, a continuing two-part need still exists in the art; namely, to provide automatic exposure control apparatus for use in a camera that not only further reduces the tedium, difficulty and guesswork associated with using currently available automated cameras to take pictures under a wide variety of different lighting conditions but also provides pictures which have an increased overall level of quality as compared to that provided by these automated cameras. Furthermore, to provide pictures at an increased quality level, a specific need exists in the art for such exposure control apparatus that automatically selects appropriate exposure settings based not only upon scene luminance but also upon scene depth-of-field requirements avoidance of image blur and, very importantly, exposure latitude and the exposure vs. quality function of the film in use.